Generally, glass materials are hard and also can be used in the form of films that coat substrates. However, when employing a melting method, a high temperature treatment is necessary for obtaining a vitreous film. This limits the materials of which the substrates and films can be made.
The sol-gel process is a process of obtaining an oxide in a solid state by: using a solution of an organic or inorganic compound of metal as a starting material; rendering the solution into a sol in which fine particles of metal oxides or hydroxides have dissolved, through the hydrolysis reaction and polycondensation reaction of the compound contained in the solution; further gelling and solidifying the sol; and heating this gel if necessary.
The sol-gel process allows vitreous films to be produced at lower temperatures. Processes of forming silica-based films by the sol-gel process are disclosed in JP55 (1980)-034258A, JP63 (1988)-241076A, JP8 (1996)-27422A, JP63 (1988)-268772A, JP2002-088304A, JP5 (1993)-85714A, JP6 (1994)-52796A, JP63 (1988)-168470A, and JP11 (1999)-269657A, for example.
Generally, the silica-based films formed by the sol-gel process have lower mechanical strength than that of vitreous films obtained by the melting method.
JP11 (1999)-269657A discloses a process for producing a silica-based film by applying an alcohol solution that is used as a coating solution to a substrate. The alcohol solution contains 0.010 to 3 wt %, in terms of silica, of at least one selected from silicon alkoxide and hydrolysate thereof (including partial hydrolysate), 0.0010 to 1.0N of acid, and 0 to 10 wt % of water.
The silica-based film obtained by this process has strength to an extent that allows the film to withstand the dry abrasion test. It probably cannot be said that the silica-based film has sufficiently high strength, but it has high mechanical strength for a film obtained by the sol-gel process. However, in the case of the silica-based film that can be formed by the process disclosed in JP11(1999)-269657A, the thickness thereof is limited to 250 nm maximum when consideration is given to obtaining an appearance that is good enough for practical use. The thickness of the silica-based film that is formed by the sol-gel process is generally around 100 to 200 nm.
As is disclosed in JP63 (1988)-168470A, a coating solution containing colloidal silica added thereto allows a film with a thickness exceeding 1 μm to be formed through a single application. However, the film obtained using this coating solution has a pencil hardness of merely around 8H and does not have sufficiently high mechanical strength.
When the coating solution is applied twice, the film obtained thereby has increased thickness. However, the silica-based film thus obtained does not have high characteristics in mechanical strength, particularly abrasion resistance. This is mainly because the coating solution is applied twice, which induces cracks in the film.
Consequently, it is difficult to obtain a silica-based film that has a thickness exceeding 250 nm and has excellent mechanical strength.
A technique of forming an organic-inorganic composite film by the sol-gel process has been proposed. The film contains an inorganic material and an organic material that are composited. The sol-gel process is characterized by film formation that is carried out at a lower temperature. Accordingly, it allows a silica-based film containing an organic material to be formed. Organic-inorganic composite films that are formed by the sol-gel process are disclosed in JP2574049B, JP2680434B, and JP2002-338304A, for example.
In order to improve the mechanical strength of the silica-based film that is formed by the sol-gel process, it is desirable to heat-treat the silica-based film at 450° C. or higher. However, when an organic-inorganic composite film is heat-treated at a high temperature of this level, the organic material in the film will decompose. The restriction that the film must be heat-treated at a temperature in the range that does not cause the organic material to decompose limits the improvement in mechanical strength of the film to be formed, not only in the sol-gel process but also in the liquid-phase film formation methods other than that. Hence, it has been considered that it is difficult to form a silica-based film that has excellent mechanical strength when the film contains an organic material.
Conventionally, it also has been considered that it is even more difficult to form a silica-based film that is thick and has excellent mechanical strength when the film contains an organic material.